1. Field of the Invention
The present invention relates to a process of making a SQUID (Superconducting Quantum Interference Device) device having a superconducting film with a high superconducting transition temperatures.
2. Description of the Prior Art
A SQUID device using superconducting film utilizes Josephson junction and is currently used as a fluxmeter for measuring an abnormal localized field and its derivative amount with increased sensitivity and accuracy (see "Technique of Josephson Computer", Parity, Separate Volume No. 1, 1986, pp 26-38).
The prior art provides SQUID devices which mainly utilize a tunnel junction including a very thin insulation film which is sandwiched between members of niobium (Nb) or niobium nitride (NbN). It is always assumed that these SQUID devices operate at boiling point of liquid helium (4.2K). If SQUID devices are formed of a superconductor of high-temperature oxide, they can operate at boiling point of liquid nitrogen, resulting in increased practicality and economical efficiency.
Presently, however, many DC-SQUID's formed of superconducting oxide films are produced by forming steps in any suitable manner before growth (see "Japanese Journal of Applied Physics", Vol. 30, No. 6B, L1121-L1124, January 1991).
On the other hand, it is known in the art that the naturally produced tilt-boundary junction functions as a Josephson junction. It has been reported that the critical current was measured in a twin tilt-boundary junction of yttrium compound (YBa.sub.2 Cu.sub.3 O.sub.7-.delta.). (See "PHYSICAL REVIEW LETTERS" issued on Jul. 11, 1988, Vol. 61, No. 2, pp 219-222.)
However, a method of controlling the production of Josephson junctions from the tilt-boundary junction is still not known.
It was further difficult to arrange a number of stepped DC-SQUID's in any pattern at desired locations.